Photomultipliers can convert the smallest light excitation signals into electrical signals in that individual electrons which are released by photons in the vacuum of the photomultiplier tube can be accelerated and at so-called dynodes can produce an avalanche of secondary electrons. In multichannel photomultipliers (MCPMT for short), the electron multiplication stages of the dynode are configured as individual channels. Each inlet region of the cathode is thus associated with an anode pin at the output. The multichannel photomultiplier functions, therefore like a multiplicity of tightly packed individual photomultipliers. A coupling of the channels rises only in that the channels formed by the dynodes are electrically connected together. The charges which arise at the isolated anodes of the channels are thus a significantly amplified image of the light distribution over the photocathode, whereby a locally precise arrangement of the light excitation is given by the channel pixels.
With multichannel photomultipliers, a detection of scintillation excitations or events can be captured with excellent spatial or positional resolution as is required, for example, in nuclear medicine. Position-sensitive detectors are known, for example in small animal PET scanners which embody such devices capable of local resolution. They use the position sensitive photo multipliers and can extract the measurement signal in accordance with center of mass principles or center of mass formation. With these photomultipliers, the amplifying electron clouds do not travel within separate channels as in multichannel photomultipliers but only the anodes are separated and usually configured in the form of a wire grid. Depending upon the position of the light event, charge levels can be differentiated on the individual wires of the grid, whereby the mutually parallel wires are collected together by means of a resistance ladder. From the ratio of the pulse intensities at the ladder ends, the location of the light event respectively in one or the other dimension of the detector surface can be determined corresponding to its center of gravity formation. The information of the total charge is delivered in the form of the sum of the four pulses.
The signal readout in accordance with the center of gravity formation principle requires therefore four analog to digital convert channels for an individual location sensitive photomultiplier. Furthermore, this principle is sensitive to disturbance signals noise since in addition to the intensity of the light pulses to be detected, the location information is coded into the pulse level variations and thus extremely small pulses can arise. Correspondingly the dynamics of the measured value acquisition must satisfy high quality requirements. Aside from this, the resistance ladders which are connected to the wire grid anode have unequal impedances at the anodes which can be the origin of locally-dependent pulse-shaped distortions. Thus problems can arise in the use of this system which can require pulse shape analysis. Furthermore, in the reading process the center of gravity formation approach may be disadvantageous since it may not be able to distinguish whether a light pulse is sharp at any given location or has been blurred or spread to simultaneously appear as if it arises from a number of locations. The latter case can arise when a gamma particle causes scattering in a scintillator which brings about a secondary scintillation at a location spaced from the initial scintillation.
Multichannel photomultipliers, of course, have separate output signals for each channel. Correspondingly, pixel-oriented measurement signals could be acquired with shape and intensity. With a large pixel count, therefore, extensive readout electronic circuitry must be provided and for the further data processing, for each channel an analog/digital converter is also required. As a result only unusually small systems with a reduced pixel count can work with such a readout method. Alternatively, the integrated pulse could be integrated by sample-and-hold stages so that they then can be read out via a multiplexer. This is however a drawback since information as to the pulse shape and the precise point in time that a pulse was created will be lost. In addition, because of the serial readout, the maximum event read which can be handled in this system is strongly limited.